1. Field
The following description relates to a Time and Wavelength Division Multiplexing-Passive Optical Network (TWDM-PON) system and a communication link method thereof.
2. Description of the Related Art
A Time Division Multiplexing Passive Optical Network (TDM-PON), such as Ethernet Passive Optical Network (EPON) and Gigabit capable PON (GPON), connects a central office and subscribers using a single upstream wavelength optical signal and a single downstream wavelength optical signal. In addition, the TDM-PON employs an optical splitter that does not require power consumption to connect a central office and subscribers. Due to these advantages, TDM-PON has been widely deployed and used worldwide. In particular, the GPON is now common across the North America and European countries. In 2010, an International Telecommunications Union Telecommunication (ITU-T) has developed G.987 XG-PON recommendation and now an early-stage commercial product has been released. In addition, Full Service Access Network (FSAN), an ITU-T SG15 for the world's leading telecommunications services providers and equipment suppliers to work toward pre-standardization, has adapted Time and Wavelength Division Multiplexing Passive Optical Network (TWDM-PON) as a main technology. ITU-T is now discussing adaption of TWDM-PON as G.989.x recommendation.
FIG. 1 is a block diagram illustrating a PON, such as TDM-PON, P-to-P, RF video overlay, which accommodates a plurality of heterogeneous services using Wavelength Division Multiplexing (WDM) within a single network. In FIG. 1, a Second Next Generation Passive Optical Networks (NG-PON2) system may be a hybrid system that combines TDM and WDM. NG-PON2 system is structured to accommodate a plurality of heterogeneous and/or homogeneous service links using optical signals of multiple wavelengths. In addition, the NG-PON2 system is able to expand a transmission capacity according to a number of optical wavelength channels without changing an Optical Division Network (ODN).
Referring to FIG. 1, a NG-PON2 system, which is a kind of a TWDM-PON system, is a hybrid PON that accommodates a plurality of central offices and n number of Optical Line Terminals (OLTs) using different wavelengths. If it is assumed that each central office accommodates a single PON link, an ODN may accommodate a number of homogeneous or heterogeneous networks and each service may be distinguished by a wavelength of a used signal. In this case, Optical Network Units (ONUs) or NG-PON2 ONUs, which are user devices of the TWDM-PON system, receive from a plurality of NG-PON2 OLTs multi-wavelength downstream optical signals that are multiplexed optical signals of different wavelengths. In addition, it is required for each ONU to select a wavelength for an upstream signal corresponding to a downstream signal in order to communicate with a particular NG-PON2 OLT. Thus, each NG-PON2 ONU needs to include a tunable transceiver which is able to select a wavelength. The tunable transceiver includes a tunable laser and a tunable receiver.
FIG. 2 is a configuration diagram illustrating a concept of a TWDM-PON system that is a main technique of a next-generation PON. In FIG. 2, it is assumed that the TWDM-PON system includes n number of OLTs that use different wavelengths, and each OLT accommodates a single PON link. In addition, one ODN may accommodate N number of TDM-PONs, and each TDM-PON link may be distinguishable by a wavelength thereof that is independent of a wavelength used by a different TDM-PON link.
In the TWDM-PON system shown in FIG. 2, each OLT which has established communication link to a different ONU may be distinguished by used wavelengths (a downstream signal wavelength and a corresponding upstream signal wavelength). For example, at least one ONU (e.g., an ONU A) may communicate with an OLT #1 using communication wavelengths (a downstream wavelength λd1 and an upstream wavelength of λu1) of the OLT #1, and, similarly, an ONU B may communicate with an OLT #2 using communication wavelengths (a downstream wavelength of λd2 and an upstream wavelength of λd2) of the OLT #2. In this case, each ONU transmits an upstream signal to an OLT using a wavelength that matches or corresponds to a downstream signal assigned thereto. Specifically, different-wavelength upstream signals are multiplexed and then transmitted within the ODN, and the multiplexed upstream signals are split by a demultiplexer (that is, a WDM MUX located in a shared infrastructure) based on wavelengths and then transmitted to corresponding OLTs, respectively.
Meanwhile, downstream signals received from the OLTs are multiplexed by a wavelength multiplexer (which is within a shared infrastructure) and then split by a splitter within the ODN, so all the downstream signals are input to or received in each ONU. In addition, each ONU uses only an optical signal of a specific wavelength among all the received multi-wavelength optical signals using a tunable receiver.
Such a TWDM-PON system includes a plurality of OLTs, and traffic imbalance may be caused according to whether an ONU being communication-connected to any OLT operates. For example, ONUs which has established communication link to a particular OLT or the OLT #1 generate heavy traffics according to a network state or time, but ONUs being communication-connected to OLTs other than the particular OLT hardly generate traffics, so that it leads to traffic imbalance between PON links. In this case, traffic loads occurs in the OLT #1, especially a PON link of a wavelength that is used by the OLT 1#, so all the ONUs being communication-connected to the OLT #1 may not receive a service with decent quality and may not utilize the entire PON links efficiently. In addition, every OLT provides a service even during the night time where traffic loads are usually significantly reduced compared to the day time, so that network resources may be unnecessarily used